Coil windings of electromagnetic machines such as stator windings of large electric generators or motors are subject to localized hot spots which must be quickly detected and diagnosed in order to prevent coil damage and to allow shutdown before failure. It is also desirable to acquire timely and detailed information concerning the severity and progression of coil stress so that decisions can be made to effect timely preventive actions. In power generation systems, such decisions may involve trade-offs affecting optimum management of electric generating capacity based on the availability and cost of replacement parts and the time that the generator must be off-line for repair.
Continuous monitoring of coil temperature is believed to be the most reliable means for detecting malfunctions in generator or motor stator windings. However, current systems use only indirect monitoring techniques of placing sensors along a coolant flow path and provide, at most, only an average measurement of the temperature variations along each coil section of a winding. Such indirect monitoring of coil temperature is in part due to a hostile coil environment requiring that sensors be remotely positioned from the coils. As a result, measured coolant temperature may be lower than the temperature of potential failure points in the coils. Given the coolant flow rate, the heat transfer rate from the conductor to the coolant, and the relatively large surface area over which the coolant flows, a coil failure may cause extensive damage before being detected by an indirect measurement technique. It is therefore desirable to provide a sensing system which utilizes one or more probes to timely and accurately monitor significant temperature increases along the entire length of each coil section. It is also desirable to provide a system which more precisely locates areas of higher temperatures which may be indicative of potential coil failure. With such a system, timely judgments and decisions can be made to minimize the costs associated with a coil failure.
Generally, probes which are useful for monitoring temperature along a line may be divided into four classes according to the type of information which they provide. Class 4 point monitors measure temperature at a specific single position. It is not believed that point monitors provide sufficient data for rapid detection and diagnosis of failures occurring along a coil winding. Class 3 distributed monitors measure the magnitude of a maximum temperature along a line. Class 2 location specific distributed monitors are capable of providing the magnitude and position at which a maximum temperature occurs along a line. Class 1 location-temperature distributed monitors are capable of measuring the temperature at a plurality of positions, thus providing a temperature profile along a line.
It is believed that distributed temperature sensing systems have not been developed in the past for monitoring coil windings because of difficulties in meeting the strict and sometimes conflicting mechanical and electrical requirements associated with generator stator environment. A distributed sensing system suitable as an on-line temperature monitor in a power plant environment must be compatible with the high voltage dielectric requirements of the stator winding insulation and must have little or no signal drift over the normal operating temperature range. It is also required that any probe used in the sensing system have a longitudinal degree of freedom along the coil length in order to avoid thermally induced strain which may alter the temperature measuring characteristics of the probe. Therefore, the probe cannot be directly bonded to either the metallic conductor or the insulating groundwall of the winding coil. Furthermore, if the probe is bonded to the groundwall, differential thermal expansion between various materials may cause a fracture of the probe. On the other hand, a distributed sensing probe which is positioned with a longitudinal degree of freedom in the insulating groundwall will introduce voids in the dielectric which lead to electrical discharge (partial discharge) and significantly shortened life of the groundwall.